1. Field of the Invention
The present invention relates to a front scattering film used for reflection type liquid crystal display units. In particular, the present invention relates to a front scattering film suitable for reflection type liquid crystal display units which have a metallic mirror electrode in a liquid crystal cell and utilize it as a reflecting board, and a front scattering film of improved optical characteristics, which can be prepared by an improved manufacturing process.
2. Related Art
Reflection type liquid crystal display units have attracted attention for their high utility in portable information terminals or video cameras having liquid crystal displays since they do not require a back light source and are very much lower in electric power consumption than conventional transmission type liquid crystal display units utilizing back light. However, the currently available reflection type liquid crystal display units, especially when used in a color display, have lower brightness and contrast than transmission type displays because of the small amount of light they can utilize.
Several means have been proposed to improve the display performance of the reflection type liquid crystal display units.
Among them, the reflection type liquid crystal display unit disclosed in Japanese Patent Unexamined Publication (KOKAI) No. 8-201802/1996 has a structure comprising a mirror reflection electrode, liquid crystal layer, transparent electrode, color filter, and transparent substrate stacked in this order on which a retardation film, polarizer film and front scattering board are further stacked in this order. It is a useful reflection type liquid crystal display device because it exhibits high contrast and does not suffer from problems of viewing angle dependency, double image and the like.
The front scattering film used for a reflection type liquid crystal display device of the above-mentioned type has a single-layer structure in which spherical particles are dispersed in the film.
In such a front scattering film, not all of the particles are embedded within the film and some of them protrude from the surface. This makes it difficult to obtain a smooth surface. The particles protruding from the surface make the surface rough and cause backward scattering.
A front scattering layer for the reflection type liquid crystal display element has been also proposed (Japanese Patent Unexamined Publication (KOKAI) No. 9-211496/1997), which is prepared by applying photosensitive resin containing microparticles dispersed therein to a substrate (film, polymer sheet, glass plate, etc.), and curing the resin by UV irradiation while imparting pressure thereto by a pressing plate to smoothen the surface.
Although this process can provide a front scattering layer with a smooth surface, it requires large-scale equipment for irradiating the resin with ultraviolet rays under pressure and, therefore, does not rank as an easy process.
In addition, since the front scattering layer is formed directly on the substrate, any defect in the front scattering layer becomes a defect of the entire unit. This may lower productivity.
The inventors of the present invention previously proposed a front scattering film having transparent resin films on both sides of a light scattering layer, which can be produced with relatively simple equipment. This front scattering film can reduce backward scattering and provide a clear display.
This front scattering film has drawbacks, however, when it is applied to another type of liquid crystal display unit in which the light scattering layer is arranged between a retardation film or polarizer film and a liquid crystal layer. Specifically, if the transparent resin film used as a substrate has optical anisotropy, it may affect polarization of light and impair the function of the liquid crystal display unit.
An object of the present invention is to provide a front scattering film, which can be produced with relatively simple equipment, reduce backward scattering and provide a clear display. Another object of the present invention is to provide a front scattering film which does not impair the function of the liquid crystal display unit by, for instance, affecting polarization, even if it is used in a liquid crystal display unit in which a light scattering layer provided by a front scattering film is arranged between a retardation film or polarizer film and a liquid crystal layer.
In order to attain the above-mentioned objects, the present invention provides a front scattering film comprising a light scattering layer composed of a transparent polymer binder and spherical microparticles dispersed in the binder and having a refractive index different from that of the binder, and substrates provided on the both sides of the light scattering layer, wherein at least one of the substrates is peelable from the light scattering layer.
A front scattering film according to one embodiment of this invention is characterized in that a ratio of the refractive index n of the spherical microparticles to that of the transparent polymer binder satisfies 0.91 less than n less than 0.99.
A front scattering film according to another embodiment of this invention is characterized in that the film has a distinctness of image of 60.0% or more.
A front scattering film according to another embodiment of this invention is characterized in that thermosetting resin or ionizing radiation curable resin is used as the transparent polymer binder.
A front scattering film according to another embodiment of this invention is characterized in that adhesive resin is used for the transparent polymer binder.
A front scattering film according to another embodiment of this invention is characterized in that at least one of the substrates is formed on the light scattering layer via an adhesive resin layer.
A front scattering film according to yet another embodiment of this invention is characterized in that at least one of the substrates is a polarizer film or retardation film
A front scattering film according to the present invention comprises a light scattering layer composed of a transparent polymer binder and spherical microparticles dispersed in the binder and having a refractive index different from that of the binder, wherein the light scattering layer is formed between two sheets of substrates and at least one of the substrates is peelable from the light scattering layer.
In the front scattering film of the present invention, the spherical microparticles do not protrude from the surface of the light scattering layer because the layer is sandwiched between the substrates. Accordingly, the surface of the front scattering film is not roughened and back scattering is reduced. Further, a front scattering film having reduced backward scattering can be produced with relatively simple equipment.
According to the present invention, a front scattering film having high contrast and capable of providing a clear display can be provided because the surface of the light scattering layer is not roughened and has a predetermined optical property.
According to the present invention, an optical property of the substrate does not affect the function of the liquid crystal display unit, since the front scattering film is built into the liquid crystal display unit after at least one of the substrates is peeled therefrom. Particularly, a front scattering film of this invention does not degrade the function of the liquid crystal display unit by, for example, affecting the polarized light, when it is applied to a liquid crystal display unit in which a light scattering layer provided by the front scattering film is arranged between a retardation film or polarizer film and a liquid crystal layer.